The present invention relates to a method and plant for burning sludge or filter cakes when manufacturing cement clinker, by which method cement raw meal is preheated, calcined and burned into clinker which is subsequently cooled, and where the sludge is dried and finely divided prior to burning.
In connection with the manufacturing of cement it is often necessary to dry and crush raw materials, e.g. clay and chalk, which are constituent parts of the cement. For this process, where the water percentage in the raw materials may be as high as 30%, it is established practice to use a drier crusher, which may be a hammer crusher, with a hot gas stream being continuously drawn through this crusher. This drying gas may either be exhaust gases from the kiln, which are normally available at temperatures around 350.degree.-400.degree. C., or air heated in a separate heat generator.
Burning of sewage sludge after the sludge has been subjected to a drying process in connection with cement manufacture is known i.a. from an article in"World cement"; March 1987; Obrist, A.;"Burning sludge in cement kilns", which describes the inherent advantages and disadvantages of burning. With regard to the burning process itself, it is stated (page 57, column 2, last section) that a treatment of the sludge is required prior to burning, particularly in order to reduce the very high water content of about 96% present in the sludge in the crude state.
A mechanical dehydration may reduce the water content to a level of 55-75%, but the sludge is still not suitable for burning, and additional thermal drying is therefore required in order to reduce the water percentage to about 5-25%, whereafter the form of the sludge is such that it can be burned in the kiln or in the calciner. The article does not mention any specific method of drying.
The Danish patent application No. 2829/90 filed on Nov. 29, 1990, suggests a method for burning paper sludge in connection with the burning of cement clinker. According to this patent specification the paper sludge is mixed with the cement raw materials in a suspension which is subsequently fed to a rotary kiln which is operating according to the wet-process method. The wet process is no longer extensively used for cement manufacturing, and prior drying and, in some cases, a fine division will be required to feed raw materials with such a high water content directly into a kiln which is operating according to the dry process. As a result, an effective, and preferably energy-saving, drying process is needed to treat the sludge before it is fed to kiln or calciner.
Current methods of drying which consists of bringing the wet sludge into contact with hot exhaust gas has the disadvantage that they result in substantial volumes of used drying exhaust gas which may contain malodorous and even toxic constituents, hence necessitating comprehensive and expensive cleaning methods in connection with the release into the atmosphere.
Oil sludge, e.g. residues from disused oil tanks, receiving stations in harbours for used engine oil and residues originating from the scrapping of automobiles etc., is a different type of sludge which must be subjected to a treatment before the calorific value of the sludge can be utilized, and for treatment of such sludge is known a mobile plant (EWOC 240, EWOC Ltd.) which, by means of calcium oxide, converts the oil sludge into a brown, free-flowing powder. During the process in this plant, burned lime is mixed into the oil sludge to heat it up, to boil off the water and to neutralize acid constituents. However, burned lime is relatively expensive, and another complication of the process is that vapour is formed which contains organic constituents which cannot be released to the atmosphere in an environmentally acceptable manner without prior treatment.